1. Field of the Invention
This invention relates to an improved process for the separation and recovery of a dimeric cyclic ester of an alpha-hydroxycarboxylic acid from vapor product streams comprising the cyclic ester and hydroxylic impurities including the alpha-hydroxycarboxylic acid. More particularly, the invention relates to a solventless distillation process for said separation and recovery. Still more particularly, the invention relates to such a solventless process wherein the vapor product stream is produced on depolymerizing an oligomer of an alpha-hydroxycarboxylic acid, the vapor product stream is condensed and fractionally distilled to recover the cyclic ester.
2. Description of Related Art
Dimeric cyclic esters of hydroxycarboxylic acids such as glycoide (1,4-dioxane-2,5-dione) and lactide (1,4-dioxane-3,6-dimethyl-2,5-dione), are intermediates to high molecular weight polyhydroxycarboxylic acids which may be useful in biomedical and other applications because of their ability to be degraded biologically and hydrolytically to form physiologically and environmentally acceptable by-products.
To achieve the high molecular weights required for such use it is necessary the cyclic ester be substantially free of hydroxylic (including hydroxycarboxylic) impurities, since such impurities prevent the attainment of the desired molecular weights. It is preferred the acid content of lactide, for example, be less than 10 milliequivalents per kilogram (meq/kg), more preferably less than 5 meq/kg.
Lactide and other dimeric cyclic esters of alpha-hydroxycarboxylic acids are most conveniently prepared by polymerizing the corresponding alpha-hydroxyacid to a relatively low molecular weight (oligomeric) polyhydroxycarboxylic acid, then heating the oligomer, generally in the presence of a catalyst, as is well known in the art, to depolymerize it to the cyclic ester (lactide) which is recovered as a component of a vapor product stream, see for example, Gruter et al., U.S. Pat. No. 1,095,205 (1914); Lowe, U.S. Pat. No. 2,668,162 (1954); Bhatia, U.S. Pat. No. 4,835,293 (1989); DeVries, U.S. Pat. No. 4,797,468 (1989); and Muller U.S. Pat. No. 5,053,522 (1991).
The vapor product stream invariably contains not only the dimeric cyclic ester but volatile hydroxylic impurities, among them water, the monomeric alpha-hydroxyacid, which is generally more volatile than the dimeric ester, and often higher boiling oligomers of the alpha-hydroxyacid, all of them undesirable as they are polymerization chain stoppers.
Further, under the conditions of typical previously known art procedures for the separation and recovery of the cyclic ester from the vapor product stream, such as condensation, scrubbing with a solvent or crystallization from a solvent, the hydroxylic impurities, particularly water and alpha-hydroxycarboxylic acid, are capable of undergoing ring-opening reactions with the cyclic ester, resulting in decrease in cyclic ester yield and increase in the acidity of the cyclic ester product. Such reactions are more prone to occur the higher temperature of the recovery process employed.
For example, removal and recovery of a cyclic ester such as lactide from the vapor product stream by scrubbing with an alcohol such as isopropyl alcohol, as exemplified in U.S. Pat. Nos. 4,835,293 and 5,053,522, not only provides a medium for potential yield-decreasing reaction of the vapor stream hydroxylic impurities with the lactide product but entails the further possibility of the hydroxylic solvent itself reacting with the cyclic ester to form yield-decreasing open-chain derivatives thereof.
Moreover, reliance on a solvent, whether for scrubbing the vapor product stream to recover the cyclic ester or for purifying it by recrystallization, is disadvantageous as it necessitates facilities for storing the solvent, using it, purifying it and preventing it from escaping into and contaminating the environment, all of which adds significantly to the process investment and operating costs.